1. Field of the Invention
The present invention relates to a semiconductor device having a circuit composed of a thin film transistor (hereafter referred to as TFT), and to a method of manufacturing thereof. For example, the present invention relates to an electro-optical device, typically a liquid crystal display panel, and to electronic equipment loaded with this type of electro-optical device as a part.
Note that, throughout this specification, semiconductor device denotes a general device which can function by utilizing semiconductor characteristics and that the category of semiconductor devices includes electro-optical devices, semiconductor circuits, and electronic equipment.
2. Description of Related Art
In recent years, techniques of structuring a thin film transistor (TFT) by using a semiconductor thin film (with a thickness on the order of several nm to several hundred of nm) formed on a substrate having an insulating surface have been in the spotlight. The thin film transistor is being widely applied in an electronic device such as an IC or an electro-optical device, and in particular, its development as a switching element of an image display device has been proceeding rapidly.
Conventionally, a liquid crystal display device is known as an image display device. Active matrix type liquid crystal display devices have come into widespread use due to the fact that, compared to passive type liquid crystal display devices, a higher definition image can be obtained. By driving pixel electrodes arranged in a matrix state in the active matrix type liquid crystal display device, a display pattern is formed on a screen. In more detail, by applying a voltage between a selected pixel electrode and an opposing electrode corresponding to the pixel electrode, optical modulation of a liquid crystal layer arranged between the pixel electrode and the opposing electrode is performed, and the optical modulation is recognized as a display pattern by an observer.
If roughly divided, two types of active matrix liquid crystal display devices are known, a transmitting type and a reflecting type.
In particular, a reflecting type liquid crystal display device has the advantage of lower power consumption compared to a transmitting type liquid crystal display device because a back light is not used, and the demand for its use as a direct view display in mobile computers and video cameras is increasing.
Note that the reflecting type liquid crystal display device utilizes an optical modulation effect of a liquid crystal, and display of light and dark is performed by selecting between a state of incident light reflected by a pixel electrode and output externally to the device, and a state of the incident light not output externally to the device, and in addition, image display is performed by combining the two states. Further, a color filter is attached to an opposing substrate in order to display colors. In general, the pixel electrode in a reflecting type liquid crystal display device is made from a metallic material having a high light reflectivity, and is electrically connected to a switching element such as a thin film transistor (hereafter referred to as a TFT).
The use of this type of active matrix type electro-optical device is spreading, and along with making the screen size larger, demands for higher definition, higher aperture ratio, and higher reliability are increasing. Further, at the same time, demands are increasing for improving productivity and lowering costs.
Conventionally, an amorphous silicon film is preferably used as an amorphous semiconductor film because of the capability of forming it on a large surface area substrate at a low temperature equal to or less than 300° C. Further, a reverse stagger type (or bottom gate type) TFT having a channel forming region formed of an amorphous semiconductor film is often used.
Furthermore, the color filters have R (red), G (green), and B (blue) coloration layers, and a light shielding mask covering only the pixel gap, and red, green, and blue colored light is extracted by transmitting light through the layers. Further, the light shielding mask is generally composed of a metallic film (such as chrome) or an organic film containing a black color pigment. By forming the color filters in positions corresponding to the pixels, the color of light output from each pixel can be changed. Note that the term positions corresponding to the pixels denotes positions coinciding with the pixel electrodes.
Conventionally, the production costs have been high in order to manufacture a TFT on a substrate with a technique of photolithography using at least 5 photomasks for an active matrix type electro-optical device. In order to improve productivity and yield, reducing the number of steps is considered to be an effective means.
Specifically, it is necessary to reduce the number of photomasks needed to produce the TFT. The photomask is used in a photolithography technique in order to form a photoresist pattern, which becomes an etching process mask, on the substrate.
By using one photomask, there are applied with steps such as applying resist, pre-baking, exposure, development, and post-baking, and steps of film deposition and etching before and after, and in addition, resist peeling, cleaning, and drying steps are added. Therefore, the entire process becomes complex, which leads to a problem.
Further, after forming the pixel electrode in the reflecting type liquid crystal display device, the surface is conventionally given unevenness by adding a step such as sand blasting or etching, preventing specular reflection and increasing the white color level by scattering reflected light.
Furthermore, in a conventional liquid crystal display panel using a metallic film as a color filter light shielding mask, a parasitic capacitance forms with other wirings, and a signal lag problem easily develops. In addition, when the organic film containing the black pigment is used as the color filter light shielding mask, a problem of an increase in the number of process steps develops.
The present invention is for answering these types of problems, and an object of the present invention is the realization of a reduction in production cost, and an increase in yield, by reducing the number of TFT manufacturing steps in an electro-optical device, typically an active matrix type liquid crystal display device.
Further, an object of the present invention is to provide a method of manufacture in which unevenness is formed for preventing specular reflection of the pixel electrode without increasing the number of process steps.